<!doctype html>
<html>

<head>
	<meta charset="utf-8">
	<title>无标题文档</title>
</head>
<style type="text/css">
	.content {
		display: flex;
		align-items: flex-start;
	}

	.left {
		width: 120px;
		background-color: #0F0;
		height: 100px;
		flex: 0 0 auto;
	}

	.right {
		background-color: #F00;
		height: 100px;
		flex: 1 1 auto;
	}
</style>

<body>
	<div class="content">
		<div class="left"></div>
		<div class="right"></div>
	</div>
</body>
</html>